During the coming year we shall continue to perfect somatic cell genetic procedures and apply these techniques to problems of biological significance. These problems include: (1) The mapping of mammalian genes, especially in man, mouse, and the Chinese hamster; (2) Analysis of control mechanisms which regulate differentiation in erythrocytes and erythroleukemias; (3) Analysis of control mechanisms which regulate differentiation in teratomas, and the use of teratomas to transfer genetic material into the murine germ line; (4) The genetic control of replication and expression of murine viruses and (5) Genetic analysis of the interferon system.